Hot mix asphalt (hereinafter “HMA”) or stone matrix asphalt (hereinafter “SMA”) is commonly used in highway construction for paving both in new road construction and in maintenance of existing roads which have surface cracks or potholes. Similarly, HMA and SMA mixes are used in airfield construction for paving both new runways and in maintenance of existing runways. These HMA and SMA mixes are currently produced in stationary plants utilizing liquid asphalt stored at temperature above the melting point typically above 250° F. to 300° F. This liquid asphalt is then pumped into a gradation of heated stone aggregated which has a specific particle size distribution properly suited for the paving or repair application. Different gradations are used for different applications, usually ranging in particle size from material passing 200 mesh (0.003 inch) to 0.75- or 1-inch diameter. About 5% to 7% by weight asphalt is mixed the aggregate to produce HMA or SMA. Plastic asphalt utilizing polymers as binders for the aggregate have also been used.
The creation and use of HMA and SMA mixes can he challenging, since both the HMA and SMA mixes and the asphalt used in the HMA and SMA mixes require the use of large amounts of energy to be maintained at relatively high temperatures until used. For example, the transport of such asphalt to be used as binder in the various asphaltic mixture applications can be problematic. Typically, asphalt is handled in bulk form as a hot liquid due to the fact that the material tends to stick together as individual particles even at room temperature when stored in the same container. During processing in anticipation for use, the asphalt must be kept heated at over 275° F. in liquid form for subsequent handling and mixing. For example, asphalt binder is handled as a hot liquid from its origination point, usually the refinery, with heated rail cars or trucks. The asphalt binder remains heated in its liquefied state through shipment to and during use in the asphalt mix plant. Since such material must be maintained at elevated temperatures for transfer to storage tanks and any transfer from one container to another until ultimate use, significant amounts of energy in the form of heat must be expended in order to maintain the asphalt in its liquefied state. It is estimated that, the United States alone, over 20 billion pounds of liquid asphalt is used each year for paving, all of which must be kept continuously heated prior to use. Depending on any additives or other materials added to the asphalt, maintaining the asphalt at elevated temperatures for prolonged periods can adversely affect the properties of the asphalt.
Similarly, problems arise in the transport of the HMA and SMA mixes. To pave roads which may be 30 or 50 miles or more from the mix plant, very careful and difficult coordination is required between the production of the hot HMA and SMA mixes at plant, the use of large trucks with consequent traffic control, the need to maintain the HMA and SMA mixes at a controlled temperature to the jobsite, and the coordination of the paving crew and equipment for proper installation of the HMA and SMA mixes for paving and patching roads, bridge decks, parking lots, airport runways, and other paving jobs. The maintenance of proper temperature of the HMA and SMA mixes is difficult to control due to the long distances the mixes are hauled. This failure of maintain the proper temperature often presents problems.
Situations which present particular difficulty using these fixed plant trucking systems are maintenance, patching, and off-season work. For example, roads, including high traffic roads, that are remote from a mixing plant may be damaged creating situations involving security and safety risks where quick local action is required. As another example, remote airfields may become damaged thus rendering them unusable. Often maintenance or pothole patching may only require a few tons of HMA or SMA at a location remote from a mixing plant. To send a truck with three to five tons of HMA from a mix plant may take several hours of expensive labor and equipment and often excess material is dumped on vacant land, creating an environmental hazard to be resolved later. This conventional system is expensive and inefficient for use for small amounts of maintenance, patching, and off-season work, especially in remote areas.
A typical drum plant used to produce asphalt today can include a direct fire continuous plant where the aggregate is introduced into a rotating drum, heated and dried to about 300° F., and liquid asphalt at about 300° F. is then introduced toward the end of the drum into the hot aggregate and mixed in the last section of the drum and then discharged as hot mix in a continuous method at about 300° F. Such a drum method can provide a continuous supply of asphalt at high throughput rates of 200 to over 500 tons per hour. The liquid asphalt, however must be kept hot continuously from the refinery, stored hot, and handled hot. Such asphalt drum mix plants have other drawbacks that exacerbate this heating problem including the facts that such plants are expensive, take time and expertise to start up, require trained personnel to operate, and are not easily moved. Further, it is difficult to meter the asphalt in a liquid in an accurate ratio generally within 1% with the continuous flow of aggregate to maintain high quality hot mix. Such processes require expensive sophisticated control systems to monitor and maintain the process.
One alternative to hot mix patching is an emulsified asphalt m applied at ambient temperature with solvents and water evaporation after placement. These “cold” mix systems are inferior in quality and must be replaced often. Also, they can be very expensive.
Another alternative uses small, portable mix units, typically 5 to 15 tons/hr, to heat recycled asphalt (hereinafter “RAP”) for patching. These units do not produce HMA or SMA of adequate quality to meet pavement grade specifications. Further, many state departments of transportation allow for only so much RAP in any given mixture of asphalts. These units normally use open flame to heat the mix which has previously been coated with binder. This open flame can degrade the RAP mix which already has the binder coating. In order to reheat the RAP mix, the aggregate which has already been coated with asphalt or polymer binders requires a large amount of heat to get the aggregate to a working temperature of 300° F. to 350° F. Such a temperature will burn or thermally degrade asphalt or polymer coating creating a substandard product which will not meet the standards or specifications of most state departments of transportation or specifications for new hot mix. Also, large amounts of smoke and unhealthy gases are produced.
To help mitigate some the problems outlined above with drum plant asphalt production as well as localized on-site small patch asphalt production, pelletized asphalt has been developed: Asphalt pellets can be manufactured by a two-step coating process which produces pellets that are over 90% virgin performance grade asphalt and the coating is mostly a fine clay which forms part of the asphalt mix design. These pellets can then be evenly mixed in a package with other aggregates to form a prepackaged asphalt mix. All materials of aggregate and asphalt are measured accurately during packaging to assure accurate formulations. These measures combined with good mixing in the field produce asphalt which is as good as or better than the virgin asphalt hot mix produced in commercial mix plants with the advantage of being capable of being stored at ambient temperatures when integrated with the aggregate and produced in minutes in remote locations upon demand. Storage and transport of the prepackaged asphalt mix is at room temperature and greatly reduces the cost of keeping the asphalt hot and reduces the degradation and separation of the asphalt from its additives over time.
The pellets are mixed with the proper aggregate mix design gradation, usually composed of 3 or 4 different gradations of aggregate from approved quarries to form a mixture of virgin asphalt and aggregate with the accurate proportions of materials for a specific mix design. This asphalt mix is placed in one ton supersacks, 5 gallon pails, or other containers and can be stored on site at ambient temperature remote locations for up to 5 years or longer. When needed for fast repairs, asphalt mix is placed in a direct or indirect fire rotating heated mixer and can produce high quality hot mix in 10 minutes or more depending on the mixer. This type of asphalt mix is currently used for repair in military airfields and other critical locations for both asphalt and concrete pavements, some of which are in remote locations that are thousands of miles from hot mix plants.
The accurate mixture of asphalt pellets and aggregate in a container such as a supersack which is transportable and storable at a remote location makes high quality repair of asphalt surfaces possible by producing hot mix on site anytime with relatively low capital cost and unskilled labor without a mix plant. However, pelletization and coating the asphalt in a fine powder coating is expensive. Therefore, the cost of producing the pelletized asphalt mix is costly. It is currently being used in small quantities for critical reapplications where material cost is not major issue.
Further, pelletized asphalt provides fast melting and mixing of the asphalt with the aggregate under heated conditions, but when storing the asphalt pellets without aggregate the asphalt pellets tend to deform and/or break the coating under gravity due to about 30% free volume and form a non-flowable mix by interlocking the pellets, especially during warm weather. The two-step pellet forming process forms a brittle coating due to the addition of powder which can crack upon deformation thus exposing the tacky asphalt which can cause pellets to stick together. A solution to the interlocking problem is to place the pellets in a gradation of aggregate in which they will be used thus filling the voids between the pellets with aggregate and thus preventing pellet deformation while keeping the mix free flowing. However, the pellets generally cannot be stored together without the aggregate for any length of time without the deformation and aggregation of the asphalt occurring. By requiring the pellets of asphalt and the aggregate to be stored together, the cost of storage and material handling associated with the prepackaged asphalt mix grows substantially due to the size and weight of the aggregate involved as the aggregate is about 95% of the weight of the total mix, while the asphalt is about 5% of the weight.
Thus, a need exists for a hot mix asphalt system that can solve such difficult problems and facilitate quick preparations of asphalt mixes with materials that can be transported and stored at ambient temperature and can be later mixed together to produce hot mix at locations near the paving site.